A nozzle retaining nut holds the two main components of a fuel injector—an injection nozzle and a valve body—tightly together. In the installed state of the fuel injector in the cylinder head, the injection nozzle projects into a combustion chamber of a motor vehicle engine, the valve body arranged above the injection nozzle actuating the injection nozzle. For this purpose it is necessary to seal the fuel injector against the cylinder head relative to the combustion chamber. This is effected by a suitable configuration of the nozzle retaining nut, which cooperates with a corresponding device, a seal seat, in the cylinder head.
High demands are placed on such a sealing arrangement. Firstly, the sealing arrangement is exposed to high thermal stresses (−40° C. when cold-starting in winter, to above +150° C. under operating conditions) and, secondly, the sealing device is subjected to high mechanical stresses, in particular vibration stresses. In addition, the sealing arrangement must ensure a permanently sealed state between fuel injector and cylinder head which can withstand long-duration stresses.
For this purpose, in the prior art, a horizontal edge, for example, is formed on the nozzle retaining nut, which seats on a likewise horizontal edge provided in the injector bore, and the nozzle retaining nut and the fuel injector are pressed against the cylinder head with a large static force. A permanently fluid-tight connection between the two edges is claimed is to be created by the provision of a large-area overlap.
With such an arrangement both sealing faces must be very accurately machined in order to obtain any lastingly fluid-tight connection. Because of the lateral clearance between fuel injector and injector bore which must be provided, centering between fuel injector and injector bore is not possible with this configuration, so that the centering must be effected by means of other arrangements or devices.
In DE 101 02 192 A1 a nozzle retaining nut has on a free end a tapered zone which is insertable in a corresponding tapered section of the injector bore. In the pre-assembled state, that is, when the fuel injector with nozzle retaining nut is inserted in the tapered injector bore, a circumferential angular difference from 2° to not more than 5° is present between the taper on the nozzle retaining nut and the tapered bore in the cylinder head. This ensures centering of the fuel injector in the injector bore, the fuel injector then being pressed into the bore with a large static force and a common sealing face being formed between the taper on the nozzle retaining nut and the conical bore in the cylinder head.
Because the angular difference between the taper on the nozzle retaining nut and the tapered bore in the cylinder head is only 2° to 5°, a large overlap area between the two sealing faces exists in the pre-assembled state, so that, because of a jamming effect between the two sealing faces, self-centering is effected in only an unsatisfactory manner. This problem is countered in the prior art by surface-coating of the corresponding surfaces, in particular that of the nozzle retaining nut, in order to reduce the coefficient of sliding friction between nozzle retaining nut and injector bore.
Despite the improvement of sliding friction between fuel injector and injector bore, the angular difference of 2° to 5° is too small to prevent jamming and to ensure self-centering. Through increasing the angular difference between the taper on the nozzle retaining nut and the tapered bore in the cylinder head to above 5°, an impairment of subsequent sealing quality would be incurred, which can lead to leakage during operation of the fuel injector.